The present invention relates to a motor driving apparatus for driving a stepper motor under chopper control.
A chopper control circuit for controlling a stepper motor is disclosed in U.S. Pat. No. 4,358,725. The two windings A1 and A2 of a stepper motor are coupled at one end. The node of windings A1 and A2 is connected to a chopper transistor coupled to a power source. The other ends of these windings are connected to two phase transistors. The chopper transistor is turned on and off by chopper control. The phase transistors are turned on and off by phase switching control.
Suppose the chopper transistor is on, the first phase transistor is on and the second phase transistor is off. In this case, an electric current flows from the power source to winding A2 through winding A1, the first phase transistor, and a current sensing transistor. This current gradually increases. When it exceeds the rated current of the stepper motor, it is detected by a current sensing circuit connected to the current sensing resistor. Upon detecting the current, the current sensing circuit supplies a signal to the chopper transistor. This signal turns off the chopper transistor, thus stopping the chopper control. Once the chopper transistor has been turned off, the current flowing through winding A1 is reduced. The magnetic field generated by winding A1 starts decaying. From the decaying magnetic field, a counter electromotive force is generated. Since windings A1 and A2 are magnetically coupled, they induce a voltage from the electromotive force. A current corresponding to this induced voltage flows through both windings A1 and A2 and through a current path constituted by diodes and resistors. This current is half the rated current of the stepper motor. Hence, the power loss at the motor is half the previous value. This current further decreases. When it becomes slightly less than 1/2 of the rated current, the current sensing circuit and the chopper control circuit turn on the chopper transistor. The chopper transistor is repeatedly turned on and off in this way as long as it is necessary to energize winding A1.
When the chopper transistor and the first phase transistor which is coupled to winding A1 are turned off to stop exciting winding A1 and excite winding A2, a current flows from winding A2 back to the power source through the diode connected in parallel to the chopper transistor.
In the chopper control circuit disclosed in U.S. Pat. No. 4,358,725, the power consumption is reduced by making the current flow through the two windings and back to the power source. However, the circuit has a drawback. Since a reference current is supplied to the current sensing circuit, the resistance of the current sensing circuit increases almost 200% when the chopper transistor is turned off and the current flowing through the windings decreases to a half of the previous value. Consequently, the accuracy of current sensing is lowered.